This invention relates to an improvement in polymer thick film (PTF) compositions containing platinum group metal catalysts, a catalyst enhancing additive, graphite or conductive carbon fillers, and a thermoplastic binder. The improved PTF conductor compositions can be used in printing sensing/working electrodes for electrochemical biosensors based on hydrogen peroxide detection. Electrochemical biosensors, which are combinations of an electrochemical sensor and a biomolecule recognition element are useful in the analysis of biological analytes such as glucose, cholesterol, creatinine, alcohol, uric acid, and lactate in body fluid, and are therefore useful in the field of medical devices and analytical instruments for medical diagnostics.
The majority of electrochemical biosensors fall within two categories (1) metal-catalyzed electrochemical biosensors or (2) electron-transfer mediator modified electrochemical biosensors. For example, a metal-catalyzed glucose sensor detects the hydrogen peroxide by-product which is produced in a one-to-one ratio from glucose through an enzyme-catalyzed air oxidation process, such as: 
U.S. Pat. No. 3,539,455 (1970) by Clark discloses a platinum based glucose sensor useful for determination of blood glucose in diabetics. Guilbault and Lubrano (1973) reported amperometric biosensors having platinum electrodes with an immobilized-enzyme suitable for glucose sensor applications. Mizutani et al. (1992) reported a platinum/carbon paste (CP) composition with a 1/9 Pt/C ratio suitable for making glucose sensors. U.S. Pat. No. 4,970,145 (1990) to Bennetto et al. discloses a biosensor with a porous enzyme electrode comprising platinized carbon paper having a fluoropolymer binder. These platinum/carbon-based biosensors have sensitivity for detection of glucose only at concentrations of millimolar (mM) glucose with an electric current response of  less than 20 xcexcA/cm2xc2x7mM glucose. U.S. Pat. No. 5,160,416 (1992) to Mullen et al. discloses an enzyme electrode produced by coating a water based suspension consisting of platinized carbon or graphite particles and enzyme. U.S. Pat. No. 5,616,222 (1997) to Maley et al. discloses a sensor working electrode comprising platinized carbon particles, enzyme, protein, and polymer binder. These enzyme-containing compositions require the coating be done at temperature well below the enzyme deactivation temperature, typically below 60xc2x0 C., and are not suitable for high throughput sensor manufacturing processes. Furthermore, these enzyme electrodes have high metal loading, typically 5-15% Pt by weight of total carbon/graphite. A working electrode with high loading of platinum group metal can lead to high material cost and unacceptably high loss of hydrogen peroxide due to metal-catalyzed decomposition of hydrogen peroxide. Patent Application WO 98/20331 disclosed ink compositions useful for printing a working electrode comprised of platinum group metal catalyst deposited on graphite and carbon black filler in a cross-linked bonded matrix. A printing ink based on a thermoset polymeric binder would require long curing time to form a cross-linked matrix, and therefore making it unsuitable for low cost high throughput sensor fabrication processes. For single-use disposable biosensors, it is critical that catalyst printing ink for the working electrode be low cost and suitable for low cost and high throughput sensor fabrication processes.
There remains a need for electrochemical sensor materials with much improved sensor performance of high catalytic activity/current response and low background current noise to expand the capability of H2O2-based biosensors for monitoring biological analytes at the micromolar (xcexcM) level and to assure a high confidence of detecting low level of analytes in body fluids. One example demonstrating the need of such a high performance biosensor was given by Tamada, Bohannon, and Potts (1995) who reported the iontophoretic extraction of body fluid, which can be used in non-invasive glucose monitoring. The body fluid can then be analyzed in situ for glucose levels and thus provide a method for non-invasive monitoring of glucose. The glucose concentration in the extracted body fluid is typically in the micro-molar level, which produces electric current in the nanoampere (nA) level, and thus requires a biosensor with low detection limit of glucose determination. A key limiting factor which affects the glucose detection limit is electrochemical signal noise, background current, which may be from electrochemically active impurities, temperature fluctuation, or from many other sources. It is desirable that biosensors have low background current, which also does not change much with temperature fluctuation.
To achieve high catalytic activity of a catalyst ink composition, catalyst particles in the composition, either as dispersed particles or as deposited particles on carbon/graphite support, may have very small size and large total surface area. Commercially available platinum metal black powders are agglomerates of nanoparticles having average particle size of  less than 100 nm. To achieve high activity, these metal powders may be further dispersed into a fine dispersion before used in a PTF ink composition. There remains a need for a process to produce very fine, stable platinum group metal dispersions having dispersed particle size of  less than 100 nm for uses in PTF compositions.
This invention is directed to a composition comprising: (a) platinum group metal powders, alloys, or mixtures thereof as a powder or deposited on graphite supports or mixtures thereof; (b) poly(glycol ether), derivatives, or mixtures thereof; (c) carbon-based electrically conductive filler; and (d) thermoplastic polymer or mixtures thereof.
The invention is further directed to a process for dispersing platinum group metal powder, alloys, or mixtures thereof comprising the steps of mixing the platinum group metal powder, alloys or mixtures thereof with poly(glycol ether), derivatives, and mixtures thereof and using a dispersing means to disperse the platinum group metal powder, alloys and mixtures thereof with the poly(glycol ether), derivatives, and mixtures thereof.
The invention is further directed to the above composition wherein the platinum group metal powders, alloys and mixtures thereof have been dispersed according to the above process.